Problem solved! (or at least symptoms gone).
I found another filter in the line under the truck. When I replaced it, the fuel from the old one had a ton of rust and stuff that dumped out.
At the same time, I looked at the HEI, and found one of the terminals and its ignition wire fouled black. So I cleaned that off.
After those two changes, it runs smooth to high speed. Not sure what solved the problem, or if I got to the bottom of the root cause, but I am happy anyway. Good to have all my horses back.
Thank you all for the help!
